Diplexers serve in radar devices to connect two inputs to one output and act, in this sense, as a frequency gate. Thus two input channels can be decoupled and separated in the signal direction, in order, for example, to be able to operate an antenna connected with one output channel. Echo signals, which are reflected from transmission signals of the antenna striking on reflecting surfaces and received back, can then be distributed back to the associated receiver. On the one hand, signals in the diplexer should be attenuated as little as possible, and, on the other hand, the decoupling between both input channels should be as great as possible. This means that signals should only be led in the desired direction, while, in the other direction, attenuation should be as great as possible.
Known diplexers for radar technology in the low cost field are, for example, arranged as microstrip couplers directly on circuit boards and can therewith be embodied very small, very precisely and very cost effectively, so that they can be used in consumer products, such as, for example, mobile telephones. Their disadvantages include low directional characteristics and high losses at moderate matching.
On the other hand, diplexers are known, whose conductor structure is formed from hollow conductor sections. A hollow conductor structure for a diplexer is composed, in principle, of two virtually parallel extending, hollow conductor channels formed of hollow conductor sections following one another, the power dividers. The hollow conductor sections of a hollow conductor channel are usually separated by a coupling zone. After the first power divider, the two power halves travel different path lengths and obtain, thus, different phases. The second power divider works as a summing element, when the two power halves, in spite of different path lengths, have equal phases. If the phases differ, however, then a weakening of the power occurs. If the phases are opposite, then the power is erased. A effective diplexer must thus be so dimensioned as regards line lengths that on the respective output line in the one frequency a summing occurs and in the other frequency a canceling. A diplexer works most effectively, when the tunable detour line fulfills the following conditions simultaneously:                the phase shift for one frequency must be 0°;        the phase shift for the other frequency must be 180°.        
The frequency separation between the two transmission frequencies is predetermined by the diplexer design. Usually, detour lines are dimensioned with a multiple of the wavelength, since then even smaller phase differences are multiplied and the diplexer obtains thereby a narrower pass-through characteristic. Therewith, the transmitting frequencies are predetermined such that the same hollow conductor length must have for the one frequency an exactly even numbered multiple and for the second frequency an odd numbered multiple of the half wavelength. The diplexer becomes, as a result, also transmissive for other frequencies, which are usually suppressed by a supplemental filter. A further reason for the use of a multiple wavelength is that therewith the installed frequency separation between the two transmitting frequencies is lessened.
FMCW-radar devices with a diplexer with a hollow conductor structure are especially suitable for broadband applications, such as, for example, distance measurement and fill level measurement in the context of industrial process measurements technology, since they are distinguished by a high power-handling capability and enable a relatively simple tuning to the desired frequencies.
Known from the publication “Compact Top-Wall Hybrid/Coupler Design for Extreme Broad Bandwidth Applications” by Ralf Beyer and Uwe Rosenberg, Microwave Symposium Digest, MTT-S International 12-17 Jun. 2005, ISBN 0-7803-8846-1/05, pages 1227-1230 is a diplexer of the above described type, which is designed for broadband use and is composed of two half shells. It uses, however, in a coupling zone between the hollow conductor sections a hollow conductor, slit coupler, which, for example, for the frequencies of greater than 50 GHz desired for industrial process measurements technology, must be relatively large, wherein it must, on the other hand, have extremely narrow coupling slits, which are difficult to manufacture.